In the jewelry industry, as well as other industries requiring precision manufactured, resilient parts, there has been a need for a gold alloy that can provide at one stage of manufacture, a highly ductile workable form, and at a second stage, deformation resistance and superior memory properties necessary to retain or return to an original shape. This is a quality especially desirable in the manufacture of springs and clasps for jewelry.
In the past, gold alloy hardness and ductility have been controlled by altering the weight percentage of silver, gold, as well as other components utilized in formulating such alloys. Grain refining components such as cobalt have been utilized in order to decrease crazing and fracture which may occur during alloy manipulation. In addition, heat treatment processes which include an annealing step to provide increased ductility, followed by an age hardening step to provide increased rigidity to gold alloys, have been utilized.
Although heat treatment processing has been useful, there has been a need to formulate a specific gold spring alloy which would optimize a heat treatment process so as to provide two different stages of alloy which exhibit maximum differences in yield strength, percentage of elongation, and tensile strength.
The relative heat treatability or hardening of a specific 10 or 14 karat alloy composition can first be estimated by use of the silver to silver plus copper ratio formula: (A. S. McDonald & G. H. Sistare, The Metallurgy of Some Carat Gold Alloys, Gold Bulletin, Nov. 1, Volume 11, 1978, pages 66 through 73) ##EQU1##
A ratio of 15% for a given alloy is considered marginally heat treatable.
If a particular gold alloy incorporates greater than a 5 weight percent of zinc, the effect is to decrease the hardenability of the alloy, specifically by reducing the immiscibility gap. In the past, gold alloys have incorporated more than 5% zinc and have had heat treatability ratios lower than 25%. These gold alloys have been limited in their ability to be easily worked into proper shape at one stage, while still providing sufficient yield strength in final form so as to provide an alloy suitable for the manufacture of springs and clasps.
An example of the limited gold alloys of the prior art is found in U.S. Pat. No. 2,169,592, which discloses a gold alloy intended to be utilized in the fabrication of jewelry. As stated in line 12 of column 4, the zinc content can reach 12 percent by weight, well above the 5% limit above which heat treatability of a gold alloy is adversely affected. At column 4, line 25 a copper weight percent of 40.45 and a silver weight percent of 7.67 are disclosed. Utilizing the silver to silver copper ratio illustrated above, the ratio can be calculated as: ##EQU2## 15.94%, as explained above would suggest a marginally heat treatable gold alloy composition. Further, the specific example illustrated also incorporates 8.71% zinc, which further limits the hardenability of the gold alloy.
U.S. Pat. No. 2,071,216 relates to the heat treatment and production of precious metal alloys. This patent is particularly concerned with the heat treatment of alloys containing platinum and palladium as well as gold.
U.S. Pat. No. 3,141,799 discloses a heat treatment technique which is commonly used in the gold products industry. This technique improves alloy hardness simply by modifying gold content and is not concerned with the balance of the alloys constituent elements. In addition, this patent discloses that more than 5% zinc is acceptable in a heat treatable gold spring alloy composition.
The above references do not disclose, nor has there been available, a gold alloy composition specifically designed to maximize a specific two step heat treatment process so as to provide a highly ductile alloy after a first step, and after a second step, excellent hardness and resistance to deformation suitable for use in the manufacture of applications requiring high strength and resiliency, such as springs and clasps.